Fiber optic temperature sensor

ABSTRACT

A fiber optic temperature sensor ( 10 ) and system employ optical (fiber  34 ) and a fiber Bragg grating ( 36 ) using non-silica materials that can withstand temperature ranges extending well above the silica-imposed limit of 1,100 degrees C. The system measures the wavelength shift of light reflected from the fiber Bragg grating ( 36 ) and converts it into a temperature value. Specific optical fibers include sapphire, which can be used at temperatures approaching 1,800 degrees C., and yttria-stabilized zirconia (YSZ), which can be used at temperature in excess of 2,300 degrees C. One specific grating employs alternating layers of YSZ, with the percentage of yttria varying in the alternating layers to achieve the desired difference of refractive index, and another grating employs alternating layers of alumina and zirconia.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority under 35 U.S.C. §119(e) of U.S.Provisional Patent Application No. 60/428,099 filed Nov. 21, 2002, thedisclosure of which is hereby incorporated by reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under contract no.N00014-97-G011 awarded by the Department of the Navy, and from the AirForce Office of Scientific Research, under contract number ______. TheGovernment has certain rights in the invention.

BACKGROUND OF THE INVENTION

The present invention relates to the field of temperature measurementdevices and techniques based on optical technology.

In many high temperature processes, it is important to have accurateknowledge of temperature, for example to maximize efficiency. This istrue for processes such as materials processing in the metal and glassindustries, and is equally true in the measurement of turbine inlettemperatures in jet engines and in stationary gas turbine power plants.However, the maximum temperatures in these processes can reach as highas 1,700 to 2,300° C. Ordinary thermocouples cannot meet therequirements for stable and accurate operation in such high-temperatureapplications.

It has been shown that temperature sensors based on optical technologymay be employed to achieve certain benefits not possessed byconventional thermocouples. An optical thermocouple includes a silicaglass fiber, one end of which terminates in a so-called fiber Bragggrating. In one known configuration, the fiber Bragg grating is composedof alternating layers of silicon nitride and silicon-rich siliconnitride. The fiber Bragg grating responds to changes in temperature bycorresponding changes in the spectral content of reflected light,specifically by a change in the optical wavelength at which peakreflectivity occurs. This response can be exploited for use in a anoptical temperature measurement system.

A measurement system can be built in which broadband optical energy istransmitted along an optical fiber toward one end at which a fiber Bragggrating is formed. The fiber Bragg grating is disposed in an environmentwhose temperature is to be measured. A broadband optical spectrumanalyzer is also coupled to the fiber to receive optical energyreflected from the fiber Bragg grating. By analyzing the output from theoptical spectrum analyzer, it is possible to determine the amount ofwavelength shift of the peak of the reflectivity characteristic, andthen to convert this peak shift into a temperature value.

Optical-based temperature measurement systems such as those describedabove have several advantages, including the ability to withstand hightemperatures and immunity from electrical noise due to theirall-dielectric construction. With respect to temperature, however,silica-based fiber and fiber Bragg gratings are generally limited to useat temperatures less than about 1,100° C. It would be desirable to havean optical-based measurement system that permits the measurement of muchhigher temperatures such as those encountered in the industrial andturbine applications described above.

BRIEF SUMMARY OF THE INVENTION

In accordance with the present invention, a fiber optic temperaturesensor and system are disclosed that achieve the benefits of opticaltemperature sensing at much higher temperatures than have heretoforebeen possible, thus enabling the accurate measuring of temperature in avariety of high-temperature applications.

The disclosed sensor and system employ optical fiber and fiber Bragggratings using non-silica materials that can withstand temperatureranges well above the silica-imposed limit of 1,100° C. In oneembodiment, the use of sapphire optical fiber enables use of the sensorat temperatures approaching 1,800° C., while an alternative sensoremploying yttria-stabilized zirconia is capable of use at temperaturesin excess of 2,350° C. These high-temperature fibers are used inconjunction with fiber Bragg gratings made of materials that can alsowithstand such temperatures. In one case, the grating employsalternating layers of yttria stabilized zirconia, with the percentage ofyttria varying in the alternating layers to achieve the desireddifference of refractive index. Alternatively, alternating layers ofalumina and zirconia can be employed.

The dynamic range of this device is extremely wide, and can be as low asliquid nitrogen temperatures. Unlike black body or pyrometer typedevices, there is no dependence upon limiting low photon flux at lowtemperatures.

Other aspects, features, and advantages of the present invention will beapparent from the Detailed Description that follows.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will be more fully understood by reference to thefollowing Detailed Description of the invention in conjunction with theDrawing, of which:

FIG. 1 is a block diagram of an optical temperature measurement systemin accordance with the present invention;

FIG. 2 is a cross-sectional view of a high-temperature optical probeused in the measurement system of FIG. 1;

FIG. 3 is a plot of representative curves of reflectance versuswavelength for a fiber Bragg grating such as used in the optical probeof FIG. 2;

FIG. 4 is a plot of representative values of wavelength peak shiftversus temperature for a fiber Bragg grating such as used in the opticalprobe of FIG. 2;

FIG. 5 is a flow diagram of a process for converting raw opticalspectrum data from an optical spectrum analyzer into a temperature valuein the measurement system of FIG. 2; and

FIG. 6 is a plot illustrating the calculation of a fine part ofwavelength shift in the process of FIG. 5.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates a temperature measurement system employing anoptical-fiber-based probe 10 disposed in a high-temperature environment12. The high-temperature environment 12 may exhibit a temperature rangefrom −200° C. to 2,350° C., the upper end of which is considerablyhigher than the maximum temperatures that may be directly measured usingconventional means. Examples of such high-temperature environments 12include material processes (such as the manufacture of ceramics), gasturbine inlet streams (such as jet engines or power plants), rocketnozzle exhaust streams, and space applications, etc.

Extending from the probe 10 is an optical fiber 14. An optical coupler16 joins the probe fiber 14 to two additional fibers 18, 20. The fiber18 carries light from a broadband light source 22 to the probe 10 viathe coupler 16, and the fiber 20 carries reflected light from the probe10 to an optical spectrum analyzer (OSA) 24, which may be for example acharge-coupled device (CCD) array. The electrical outputs of the OSA 24are coupled to a digital processor 26.

The broadband light source 22 can be implemented by a LED or othersuitable broadband source. The range of optical wavelengths from thesource 22 encompasses a range of reflectance frequencies of a fiberBragg grating employed within the probe 10, which is described in moredetail below.

FIG. 2 shows the probe 10 in detail. The optical fiber 18 is encased ina flexible metal jacket 27 and extends into a probe body including anouter sleeve 28 of ceramic or metal, an elongated inner ceramic sleeve30, and an inner quartz sleeve 32. The ends of the probe body are sealedwith high temperature cement 34. The optical fiber 18, which istypically silica, is butt-joined to a tip optical fiber 34 of a materialcapable of withstanding extremely high temperatures. Examples of such amaterial include sapphire and yttria-stabilized zirconia. Preferably thefibers 18 and 34 are coupled using an anti-reflective coating to reduceundesirable optical reflections and losses.

Formed at the distal end of the tip optical fiber 34 is a 1/4-wavelengthfiber Bragg grating 36, which is used as a wavelength-selectivereflector. The grating can be made using different types of ceramicsystems. In one scheme, the grating 36 is made using yttria-stabilizedzirconia, with alternating layers having different concentrations ofyttria to achieve the small difference of refractive index that isrequired for a narrow reflecting structure. The percentage of yttriadoping can be from, typically, 5% to 40%. This structure retains itschemical stability when subjected to temperatures as high as 2400° C.Also, the thermal expansion properties of such layers are well matched,minimizing destructive thermal-induced mechanical strain. This isextremely important.

As an alternative, alternating layers of alumina and zirconia can beemployed. It may be desirable to add yttria to the zirconia layers toimprove the refractive index matching between the two layers. A layerhaving 20% yttrium has a refractive index of 1.9, which is close to therefractive index of 1.76 of alumina.

The grating 36 can be formed using a process in which a layer isdeposited at the end of the fiber 18 while the reflectance at aparticular wavelength is monitored. The reflectance will vary between amaximum and a minimum as each layer is deposited. When a peak or valleyof the reflectance is reached during the deposition of one layer, thedeposition is stopped and the deposition of the next layer is begun.This process is repeated until the desired number of layers have beendeposited.

Additionally, it is possible to form the grating 36 using othercombinations of repeating sequences of materials of different refractiveindices that will provide high reflectivity over a narrow wavelengthregion.

FIG. 3 generally illustrates the variation of reflectance withtemperature of a fiber Bragg grating such as grating 36. The particularcurves shown in FIG. 3 are representative of a fiber Bragg gratingemploying alternating layers of silicon nitride and silicon-rich siliconnitride, but it is expected that similar results will be obtained forfiber Bragg gratings of the type described above.

As shown in FIG. 3, the reflectance of the grating at a giventemperature will exhibit a peak at a particular wavelength. In FIG. 3,the peak reflectivity is about 84%. The horizontal location of this peakwill shift as the temperature of the grating changes. This is shown inFIG. 3 as a horizontal shifting of the reflectance-versus-wavelengthcurve. It is also shown in FIG. 4 as a scatter plot of peak shift versustemperature, under conditions of heating as well as cooling. Thevertical units of FIG. 4 are CCD pixels in the OSA 24. It will beobserved from FIG. 4 that the dependence of peak shift on temperature isalmost linear, and exhibits almost no hysteresis. In the example shownin FIG. 3, the peak occurs at about 840 nm at 25° C., and shifts toapproximately 855 nm at 1100° C. By measuring the amount of the peakshift from some predetermined calibrated position, the temperature ofthe grating, and thus of the environment immediately surrounding thegrating, can be accurately determined.

FIG. 5 shows a process for obtaining temperature measurements from theprobe 36 based on the peak shift of reflected light. In step 38, theprobe 36 is placed in an environment of known temperature, and thecharacteristic spectrum data is obtained from the OSA 24, normalized,and saved as a reference spectrum. This normalization takes thefollowing form:$\overset{\leftrightarrow}{Y} = \frac{{\left( {N + 1} \right)\overset{\leftrightarrow}{X}} - {\sum\limits_{i = 0}^{N}{xi}}}{\sqrt{\left. {\sum\limits_{i = 0}^{N}\left( {{\left( {N + 1} \right){xi}} - {\sum\limits_{i = 0}^{N}{xi}}} \right)} \right)^{2}}}$where X represents the raw spectrum data vector and Y represents thenormalized data vector. To facilitate subsequent processing, only themain portion of the spectrum containing the peak is utilized. Thisvector can be represented asA=[a _(i) , a _(i+1), . . . , a_(i+N))

In step 40, measured spectrum data is obtained at an unknown temperaturebeing measured, and this data is normalized using the same normalizationfunction described above. To facilitate the analysis steps to follow,the normalized measured spectrum data is saved as an array ofsub-vectors of the overall vector output of the OSA 24. These can berepresented as follows: B₀ = [b_(i), b_(i + 1), …  , b_(i + N)) ⋮B_(k) = [b_(i + k), b_(i + k + 1), …  , b_(i + k + N)) ⋮B_(m) = [b_(i + m), b_(i + m + 1), …  , b_(i + m + N))where m represents an assumed maximum pixel shift of the measuredcharacteristic spectrum, which corresponds to the highest temperature tobe read by the probe 36.

At step 42, the “whole” part h of the spectrum peak shift (in integernumber of pixels or CCD elements) is determined using a least squaresalgorithm on the reference and measured spectrums. This involvescomputing a measure of the difference between the normalized referencespectrum vector and each of the normalized measured spectrum vectors,and then determining which of the computed difference values is thesmallest. This algorithm can be expressed as follows:

1. For k=0 to k=m, calculate: $\begin{matrix}{d_{k} = {\left( {A - B_{k}} \right)*\left( {A - B_{k}} \right)}} \\{= {\sum\limits_{n = 0}^{N}\left( {a_{i} - b_{i + k + n}} \right)^{2}}}\end{matrix}$

2. Find the minimum d_(k), which is denoted d_(h). The value h is thewhole part of the peak shift.

In step 44, the fractional part t of the peak shift is determined. Thispreferably uses an “extreme value” calculation, which is described withreference to FIG. 6. FIG. 6 shows the relationship of several valuesused in the calculation, namely a_(i), b_(i), a_(i+1), b_(i+1), etc. Thecalculation uses the following equation:$t = \frac{\sum\limits_{n = 0}^{N}\left\lbrack {\left( {a_{i + n} - b_{i + h + n}} \right)\left( {b_{i + h + n + 1} - b_{i + h + n}} \right)} \right\rbrack}{\sum\limits_{n = 0}^{N}\left( {b_{i + h + n + 1} - b_{i + h + n}} \right)^{2}}$

Finally, in step 46, the spectral shift is calculated asS _(shift) =W _(pixel) *S _(pixel),whereS _(pixel) =h+t

and W_(pixel) is equal to the per-pixel spectral width of the OSA 24. Iflinearity is assumed, the value W_(pixel) can be calculated by dividingthe total spectral width of the OSA 24 by the number of pixels (CCDelements) in the array.

The value S_(shift) can then be translated to a temperature using apre-computed conversion factor obtained during a calibration process.This factor has units of degrees/(nm of wavelength), and thus yields atemperature in degrees when multiplied by S_(shift). In one type ofcalibration process, the steps of FIG. 5 are performed at twotemperatures of known separation, and the conversion factor is thencalculated by dividing the known temperature separation by the value ofS_(shift) that is obtained in the measurement process. For example, areference measurement can be taken at 25° C., and a second measurementtaken at 50° C., providing a known 25° C. difference in temperature.This value is divided by the value of S_(shift) obtained for the secondmeasurement to obtain the conversion factor. It will be appreciated thatother techniques for obtaining a conversion factor or a set ofconversion factors to be used for temperature measurements can beemployed, which might account for non-linearities in thetemperature-vs.-wavelength characteristic of the system.

As an example of the use of the conversion factor, if it is assumed thatthe conversion factor is 15° C. per nm, then a value of S_(shift)=37.6yields a measured temperature T of $\begin{matrix}{T = {25 + {(15)(37.6)}}} \\{= {589{^\circ}\quad{C.}}}\end{matrix}$

It will be apparent to those skilled in the art that modifications toand variations of the disclosed methods and apparatus are possiblewithout departing from the inventive concepts disclosed herein, andtherefore the invention should not be viewed as limited except to thefull scope and spirit of the appended claims.

1. A fiber optic tmperature sensor for measuring temperatures in ameasurement range from less than −200° C. to substantially beyond about1,100° C., comprising: a rigid sensor body of a heat-dissipatingmaterial; a hollow tip member extending from the sensor body, the hollowtip member being made of a material capable of withstanding temperaturesin the measurement range; and an optical fiber disposed within the tipmember, the optical fiber being made of a material capable ofwithstanding temperatures in the measurement range, the optical fiberterminating in a selectively reflective fiber Bragg grating made ofmaterials capable of withstanding temperatures in the measurement range.2. A fiber optic temperature sensor according to claim 1, wherein theoptical fiber comprises sapphire.
 3. A fiber optic temperature sensoraccording to claim 1, wherein the optical fiber comprises zirconia.
 4. Afiber optic temperature sensor according to claim 3, wherein thezirconia is stabilized with yttria.
 5. A fiber optic temperature sensoraccording to claim 1, wherein the fiber Bragg grating comprises layersof yttria-stabilized zirconia, wherein alternating layers have differentconcentrations of yttria to provide a desired difference of refractiveindex.
 6. A fiber optic temperature sensor according to claim 1, whereinthe fiber Bragg grating comprises alternating layers of alumina andzirconia.
 7. A fiber optic temperature sensor according to claim 1,wherein the tip member comprises ceramic.
 8. A fiber optic temperaturesensor according to claim 1, wherein the sensor body comprises a metalsleeve from which the tip member extends.
 9. A fiber optic temperaturesensor according to claim 8, wherein the metal sleeve and the tip memberof the sensor body are attached together by high-temperature cement. 10.A fiber optic temperature sensor according to claim 8, wherein the metalsleeve comprises copper.
 11. A fiber optic temperature sensor accordingto claim 1, wherein the optical fiber is a first optical fiber, andfurther comprising a second optical fiber having one end disposed withinthe sensor body and optically coupled to the first optical fiber.
 12. Afiber optic temperature sensor according to claim 11, wherein the secondoptical fiber is butt-joined to the first optical fiber with ananti-reflective coating interposed therebetween.
 13. A fiber optictemperature sensor according to claim 11, wherein the second opticalfiber comprises silica.
 14. A fiber optic temperature sensor accordingto claim 11, wherein the second optical fiber is disposed within arugged jacket, and wherein the jacket is disposed within the sensor bodyin a manner retaining the second fiber within the sensor body.
 15. Afiber optic temperature sensor according to claim 14, wherein the metalsleeve and the tip member of the sensor body are attached together byhigh-temperature cement.
 16. A system for measuring temperatures in ameasurement range from less than −200° C. to substantially beyond about1,100° C., comprising: a fiber optic temperature sensor having a tipportion with an optical fiber therein, the optical fiber being made of amaterial capable of withstanding temperatures in the measurement range,the optical fiber terminating in a fiber Bragg grating made of materialscapable of withstanding temperatures in the measurement range and havingreflectivity which is a function of wavelength of incident light; abroadband light source being optically coupled to the optical fiber totransmit light along the optical fiber toward the fiber Bragg grating;an optical spectrum analyzer optically coupled to the optical fiber toreceive light reflected from the fiber Bragg grating back into theoptical fiber; and a processor operative to receive one or moreelectrical signals from the optical spectrum analyzer representing theintensity of the reflected light across an optical spectrum including anoptical wavelength at which an optical characteristic of the fiber Bragggrating is detected, the processor being further operative to determinea value of the optical wavelength at which the optical characteristic ofthe fiber Bragg grating is detected and to convert the determinedwavelength value to a temperature value according to predeterminedconversion criteria.
 17. A temperature-measuring system according toclaim 16, wherein the optical fiber comprises sapphire.
 18. Atemperature-measuring system according to claim 16, wherein the opticalfiber comprises zirconia.
 19. A temperature-measuring system accordingto claim 18, wherein the zirconia is stabilized with yttria.
 20. Atemperature-measuring system according to claim 16, wherein the fiberBragg grating comprises layers of yttria-stabilized zirconia, whereinalternating layers have different concentrations of yttria to provide adesired difference of refractive index.
 21. A temperature-measuringsystem according to claim 16, wherein the fiber Bragg grating comprisesalternating layers of alumina and zirconia.
 22. A temperature-measuringsystem according to claim 16, wherein the optical fiber is a firstoptical fiber, and further comprising a second optical fiber operativeto optically couple the temperature sensor to the light source and theoptical spectrum analyzer, the second optical fiber having one enddisposed within the temperature sensor and optically coupled to thefirst optical fiber.
 23. A temperature-measuring system according toclaim 22, wherein the second optical fiber is butt-joined to the firstoptical fiber with an anti-reflective coating interposed therebetween.24. A temperature-measuring system according to claim 22, wherein thesecond optical fiber comprises silica.
 25. A temperature-measuringsystem according to claim 22, further comprising an optical couplerhaving one bidirectional optical port coupled to the second opticalfiber, the optical coupler having a light input optical port coupled tothe light source and a light output optical port coupled to the opticalspectrum analyzer.
 26. A temperature-measuring system according to claim16, wherein the optical spectrum analyzer comprises a photodetectorarray.
 27. A temperature-measuring system according to claim 26, whereinthe photodetector array comprises a charge-coupled device array.
 28. Atemperature-measuring system according to claim 16, wherein the opticalcharacteristic is peak reflectivity.
 29. A temperature-measuring systemaccording to claim 16, wherein the processor is operative whendetermining the value of the optical wavelength at which the opticalcharacteristic of the fiber Bragg grating is detected to: i) obtain andnormalize measured spectrum data from the optical spectrum analyzer whenthe system is operating at a measurement temperature; and ii) compute anamount by which the normalized measured spectrum data must be shifted inwavelength to yield shifted normalized measured spectrum data in whichthe optical characteristic is most similar to the same opticalcharacteristic in pre-established reference spectrum data.
 30. Atemperature-measuring system according to claim 29, wherein computingthe amount by which the normalized measured spectrum data must beshifted comprises (i) calculating a difference function of the referencespectrum data and each of shifted versions of the normalized measuredspectrum data, and (ii) identifying one of the shifted versions of thenormalized measured spectrum data for which the calculated functionyields a minimum value.
 31. A temperature-measuring system according toclaim 30, wherein the difference function comprises a least squaresfunction.
 32. A temperature-measuring system according to claim 29,wherein computing the amount by which the normalized measured spectrumdata must be shifted comprises (i) determining a whole part representingan integer number of shift units, (ii) determining a fractional partrepresenting a fraction of a shift unit, and (iii) adding the whole andfractional parts together.
 33. A temperature-measuring system accordingto claim 32, wherein determining the whole part comprises computing aleast squares difference function of the reference spectrum data andeach of shifted versions of the normalized measured spectrum data, anddetermining the fractional part comprises computing an extreme valuefunction of the reference spectrum data and one of the shifted versionsof the normalized measured spectrum data for which the least squaresfunction yields a minimum value.
 34. A temperature-measuring systemaccording to claim 16, wherein the predetermined conversion criteriacomprises a multiplicative factor representing a temperature differenceper unit of shift.
 35. A temperature-measuring system according to claim34, wherein the multiplicative factor is determined by a calibrationprocess that includes obtaining measured spectrum data at a knowntemperature different from the reference temperature, and dividing thedifference between the known temperature and the reference temperatureby an amount by which normalized spectrum data obtained at the knowntemperature must be shifted in wavelength to yield shifted normalizedspectrum data in which the optical characteristic is most similar to thesame optical characteristic in the reference spectrum data.
 36. In atemperature measurement system employing a fiber optic temperaturesensor and an optical spectrum analyzer optically coupled to thetemperature sensor, wherein the temperature sensor produces reflectedlight across an optical spectrum including an optical wavelength atwhich an optical characteristic of the temperature sensor can bedetected, and wherein the optical spectrum analyzer is operative toproduce electrical signals representing the intensity of the reflectedlight from the temperature sensor across the optical spectrum, a methodof generating a measured temperature value based on the electricalsignals, comprising: establishing reference spectrum data from theelectrical signals when the system is operating at a predeterminedreference temperature; obtaining and normalizing measured spectrum datafrom the electrical signals when the system is operating at ameasurement temperature; computing an amount by which the normalizedmeasured spectrum data must be shifted in wavelength to yield shiftednormalized measured spectrum data in which the optical characteristic ismost similar to the same optical characteristic in the referencespectrum data; and using pre-established conversion criteria to convertthe computed shift amount to the measured temperature value.
 37. Amethod according to claim 36, wherein computing the amount by which thenormalized measured spectrum data must be shifted comprises (i)calculating a difference function of the reference spectrum data andeach of shifted versions of the normalized measured spectrum data, and(ii) identifying one of the shifted versions of the normalized measuredspectrum data for which the calculated function yields a minimum value.38. A method according to claim 37, wherein the difference functioncomprises a least squares function.
 39. A method according to claim 36,wherein computing the amount by which the normalized measured spectrumdata must be shifted comprises (i) determining a whole part representingan integer number of shift units, (ii) determining a fractional partrepresenting a fraction of a shift unit, and {iii} adding the whole andfractional parts together.
 40. A method according to claim 39, whereindetermining the whole part comprises computing a least squaresdifference function of the reference spectrum data and each of shiftedversions of the normalized measured spectrum data, and determining thefractional part comprises computing an extreme value function of thereference spectrum data and one of the shifted versions of thenormalized measured spectrum data for which the least squares functionyields a minimum value.
 41. A method according to claim 36, wherein thepre-established conversion criteria comprises a multiplicative factorrepresenting a temperature difference per unit of shift.
 42. A methodaccording to claim 41, wherein the multiplicative factor is determinedby a calibration process that includes obtaining measured spectrum dataat a known temperature different from the reference temperature, anddividing the difference between the known temperature and the referencetemperature by an amount by which normalized spectrum data obtained atthe known temperature must be shifted in wavelength to yield shiftednormalized spectrum data in which the optical characteristic is mostsimilar to the same optical characteristic in the reference spectrumdata.